Energy seal for microwave ovens



Nov. 7,1967 K. PAHLMAN I 3, v ENERGY SEAL FOR MICROWAVE OVENS FiledMarch 9, 1965 INVENTOR. KURT c. v PA HLMAN BY 224.04%, AGET UnitedStates Patent 8 Claims. c1. 219-1055 ABSTRACT OF THE DISCLOSURE Amicrowave oven having means for sealing the oven doorway to prevent theleakage of microwave energy comprises, providing opposed recesses in themating surfaces of the door and the oven wall surrounding the ovendoorway, each of which is one-fourth wavelength'deep at the frequency ofthe microwave. energy.

The present invention relates to a sealing arrangement utilized inmicrowave stoves for preventing leakage of .microwave energy between astove door and the edge portion of the oven wall surrounding the dooropening in the stove.

In microwave stoves it is absolutelyessential that the leakage of highfrequency energy about the edge .of the door must be eliminated, orlimited to a very small amount, and high requirements are therefore laidupon the sealing of these edges. A further problem arisesbecause, due tomechanical tolerances, the micro-switches which automatically disconnectthe current energizing the high frequency source when the door is openedmay, under unfavourable circumstances, not be actuated until anappreciable gap has appeared between the door and the said edgeportions. Therefore, it is further required that the sealing means beeffective also at relatively great gaps and that the seal be relativelyin sensitive to variations in the said gap. The available space at theedge of the door for achieving a tight seal is limited and it is also arequirement that the sealing means have small dimensions.

Many different proposals for sealing the edges of the stove door inmicrowave stoves have been made. Thus it has been proposed to produce agood galvanic contact between the closed door and the edges surroundingthe opening. Such a galvanic contact is effective but in practice it isdifiicult to achieve. If, for example, due to dirt or of some otherreason, a gap should arise at some place, sparking will result with aconsequent decrease of the scaling properties. Constructions with lossymaterials around the edge of the door are known which produce asufiicient sealing of the closed door, but the dimensions of thesesealing means will be relatively great.

The invention has for an object to produce a simple and small sealingdevice for microwavestoves which also is effective over a largefrequency range and has large mechanical tolerances, in particular withrespect to the size of the gap between the stove door and the edgeportions around the stove opening.

According to the invention, an improved seal is achieved by providingsubstantially equal recesses in the electrically conductive material andhaving a depth equal to approximately wave length of the microwaveenergy coming from the inside of the stove, and appearing in the saidrecesses. The recesses are arranged in opposite portions of the edge ofthe stove door and the edge portions around the opening of the stove sothat the recesses are open against each other and situated opposite eachother in the closed condition of the door.

3,351,730 Patented Nov. 7, 1967 Experiments have shown that a sealingdevice constructed in this way is very effective and fulfills the saidrequirements to a very great extent. The device is particularlyadvantageous due to its small dimensions and also functions well atrelatively great gaps between the stove door and the edge portions ofthe side wallof the stove surrounding the opening. If itis required tofurther decrease the dimensions of the device the said recesses may befilled with a dielectric materials serving at the same time to preventcollection of dirt in the recesses.

The invention will now be described more fully in connection with theaccompanying drawing which shows a sectional view through a portion of amicrowave stove with a sealing device according to the invention.

In the drawing, reference numeral 1 designates a side wall enclosing theheating cavity 2 of a microwave stove. The front side of the'stove isprovided with a-rectangular opening which can be closed by means of adoor 3. This consists of a door frame 4 extending around the opening anda thin metal sheet 5 covering the rectangular opening formed within thesaid door frame. The metal sheet 5 may suitably be perforated in orderto make possible observation of the food situated within the stove.

According to the invention, both the door-and the-portions of the sidewall'of thestove surrounding theopening are provided with recesses 6 and7, respectively, which formed. The recesses t6, 7 are filled with asuitable dielectric material, as araldite or the like. On the one hand,the dielectric decreases the dimensions of the recesses, and on theother hand it prevents the collection of dirt within the recesses. In apractical embodiment the said recesses 6 and 7 each have a depth of 20mm. corresponding substantially to a quarter of a wave length of thehigh frequency energy coming from the inside of the stove, account beingtaken of the filling material within the recesses. The width of therecesses amounts in the said embodiment to 16 mm.

The door is mechanically actuated so that with the door closed, gaps 8and 9, respectively, of some millimeters is formed both at the inputside of the cavity 6, 7 and at the output side of the said cavity, asseen in the direction of propagation of the leaking energy. The gap 8 atthe input side should not be made too small, because sparking then mayappear between opposite surfaces of the. side wall 1 and the door 3 dueto the electromagnetic energy which is applied from the inside of thestove.

In operation the said cavity 6, 7 acts as a very high impedance for thesubstantially plane-polarized electromagnetic energy coming from thestove. As a result, a very small quantity of energy can pass through thegap 8. Due to the fact that both the depth and the width of the recesses6, 7 is approximately equal to a quarter of a wave length, the describedsealing device may be regarded as a combination of a quarter wave stub(small short circuited wave guide section) and a quarter wavetransformer. This is one of the reasons for the effective sealingproperties of the device. Measurements have furthermore shown that thedevice is effective over a very wide frequency range.

In order to produce a further barrier for leaking high frequency energy,in particular when opening the stove door, in which case the gap mayincrease appreciably before the high frequency source is disconnected, arecess 10 containing a lossy material may suitably be arranged beyondthe said cavity 6, 7 as seen in the direction of energy propagation. Therecess 10 with lossy material may possibly be missing at that side wherethe door hinges are situated because the gap at this side cannotincrease appreciably before the high frequency source is disconnected.

What is claimed is:

1. A microwave oven comprising a metallic walled heating chamber havingan access aperture in one wall of said chamber, a metallic door adjacentto and overlapping said aperture to provide a pair of spaced confrontingplanar surfaces defining a gap about the periphery of said aperture inthe closed position of the door, said one wall having a recess thereinextending about the periphery of said access aperture, said door havinga recess therein extending about the periphery of the surface facingsaid wall and in confronting relationship with said wall recess, thedepth of each of said recesses being approximately one-fourth wavelengthat the frequency of the oven microwave energy.

2. An oven as described in claim 1 wherein the width of each of saidrecesses is approximately one-fourth wavelength at the frequency of theoven microwave energy.

3. An oven as described in claim 1 wherein said recesses are filled witha low-loss dielectric material.

4. An oven as described in claim 2 wherein said recesses are filled witha low-loss dielectric material.

5. An oven as described in claim 21 wherein one of said confrontingsurfaces includesa second recess filled with a lossy dielectric materialand extending coaxially about a substantial portion of the first recessin said one surface.

6. An oven as described in claim 4 wherein said confronting surface ofthe door includes a second recess filled with a lossy dielectricmaterial and extending coaxially about a substantial portion of thefirst recess in said door.

7. A microwave oven comprising a metallic walled structure defining aheating chamber having an access aperture in one wall thereof, a dooradapted to 'close said aperture and having a conductive surfacecompletely covering and overlapping said aperture but spaced from thesurface of said one wall around the opening so as to provide a gapbetween said door and said one wall in the closed position of the door,said one wall having an annular channel therein extending around theperiphery of said access aperture, said door having an annular channelin the surface thereof facing said one wall and extending in opposedrelationship to said wall channel to form therewith a continuous cavityextending about the access aperture, the depth of each of said channelsbeing approximately one-fourth wavelength at the frequency of the ovenmicrowave energy.

8. An oven as described in claim 7 wherein the width dimension of saidchannels are identical and are approximately equal to one-fourthwavelength at the frequency of the oven microwave energy, said doorhaving a second annular channel in the surface facing said one wall andextending coaxially around the first door channel, said confrontingchannels being filled with a low-loss dielectric material and saidsecond channel being filled with a lossy dielectric material.

, References Cited UNITEDSTATES PATENTS 3,182,164 5/1965 Ironfield219-10.55

3,196,242 7/1965 De Vries et al 219-1055 3,219,747 11/1965 McAdams21910.55 X

3,249,731 5/1966 Johnson 210-1055 FOREIGN PATENTS 1,044,455 9/1966 GreatBritain.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

1. A MICROWAVE OVEN COMPRISING A METALLIC WALLED HEATING CHAMBER HAVINGAN ACCESS APERTURE IN ONE WALL OF SAID CHAMBER, A METALLIC DOOR ADJACENTTO AND OVERLAPPING SAID APERTURE TO PROVIDE A PAIR OF SPACED CONFRONTINGPLANAR SURFACES DEFINING A GAP ABOUT THE PERIPHERY OF SAID APERTURE INTHE CLOSED POSITION OF THE DOOR, SAID ONE WALL HAVING A RECESS THEREINEXTENDING ABOUT THE PERIPHERY OF SAID ACCESS APERTURE, SAID DOOR HAVINGA RECESS THEREIN EXTENDING ABOUT THE PERIPHERY OF THE SURFACE FACINGSAID WALL AND IN CONFRONTING RELATIONSHIP WITH SAID WALL RECESS, THEDEPTH OF EACH OF SAID RECESSES BEING APPROXIMATELY ONE-FOURTH WAVELENGTHAT THE FREQUENCY OF THE OVEN MICROWAVE ENERGY.